A conventional optical fiber is illustrated in FIG. 1. Shown in cross section, a first main medium 5c, having a predetermined refractive index, is surrounded by a second main medium 6c, having a refractive index lower than that of the first main medium. The core region 2c includes the first main medium, so that a lightwave localizes in the core and propagates over the fiber. The refractive index of the first main medium is typically spatially uniform, while other well-known profiles such as W-shaped profile are possible.
The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air. A cladding structure may have a spatially uniform average refractive index that can be adjusted to meet a desired relationship with the core index. As described in a paper of J. Broeng et al., published in Optical Fiber Technology, Vol. 5, pp. 305-330 (1999), page 316, with microstructured optical fibers having sufficiently large air holes, it is possible to realize lower bending losses than the conventional optical fibers.
Optical fibers that are to be wired between optical components (fibers, waveguide circuits, modules including them, etc) need to be spliceable with low loss and low cost, to be operable under small-diameter bends, and to have low multi-mode noise. Bending loss due to small-diameter bends and coupling loss due to inaccurate positioning of fiber elements are common problems. With microstructured optical fibers having spatially uniform average refractive index cladding, it has been difficult to obtain a mode-field diameter that is sufficiently large for low-loss splicing but not so large as to cause optical loss due to leakage.